**Core Concept**
The standard error (SE) of a mean is a statistical measure of the variability or dispersion of the sample mean from the true population mean. In this context, we are estimating the standard error of the mean hemoglobin (Hb) levels in a population of pregnant females.

**Why the Correct Answer is Right**
The standard error (SE) can be calculated using the formula: SE = σ / √n, where σ is the standard deviation and n is the sample size. In this case, the standard deviation of Hb levels is given as 1 gm%, and the sample size is 100 women. Plugging these values into the formula, we get: SE = 1 / √100 = 1 / 10 = 0.1 gm%.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the calculation of standard error.
* **Option B:** This option is incorrect because it is not the correct formula for calculating standard error.
* **Option C:** This option is incorrect because it does not take into account the sample size.

**Clinical Pearl / High-Yield Fact**
When calculating the standard error, remember to always square the sample size in the denominator. This is a common trap in statistics, and being aware of it can help you avoid errors in your calculations.

**Correct Answer:** . 0.1 gm%